PreferencesData: Recreate PreferencesMap in init

Normally, init is only called once during startup, so this does not add
anything. However, when running the testsuite, PreferencesData could be
initialized multiple times in a single test run. To prevent preferences
from a previous test from interfering with subsequent tests, always
start with a clean slate when calling init.

arduino-core/src/processing/app/PreferencesData.java

@@ -50,6 +50,9 @@ public class PreferencesData {
       //ignore
     }
 
+    // Start with a clean slate
+    prefs = new PreferencesMap();
+
     // start by loading the defaults, in case something
     // important was deleted from the user prefs
     try {